Orthogonal frequency division multiplexing (OFDM) has received considerable attention for its robustness against inter-symbol interference (ISI) and impulse noise, low implementation complexity and high spectral efficiency. It was first standardized for Digital Audio and Video Broadcasting applications, and later for digital subscriber loops (DSL) and wireless LAN. One important advantage of an OFDM system is its simple receiver structure utilizing a frequency domain equalizer with only one complex multiplication per sub-carrier. This is achieved by inserting a time domain cyclic prefix (CP) in front of each OFDM symbol, enabling the receiver to separate a steady-state response from a transient response of the communications channel. The CP, which is a cyclic extension of the inverse discrete Fourier transformation (IDFT) output, has to be at least as long as the channel impulse response (CIR) in order to avoid inter-symbol interference. Therefore, redundancy is unavoidably introduced into conventional OFDM systems. This restricts achievable bandwidth efficiency, especially for channels with a very long CIR.
To mitigate this problem, many OFDM receivers apply a finite-impulse response (FIR) time domain equalizer (TEQ) before the discrete Fourier transform (DFT) in order to shorten the effective length of the CIR. However, this significantly undermines the major advantage of OFDM, i.e., the simple frequency domain equalization.
Further, conventional OFDM transmission is known to be sensitive to synchronization errors, represented by frequency and timing offsets. Frequency offset at the receiver introduces inter-carrier interference (ICI) due to the loss of orthogonality among demodulated sub-carriers. Timing offset results in a rotation of the OFDM sub-carrier constellation. As a result, an OFDM system cannot recover the transmitted signal without a near perfect synchronization, especially when a high-order quadrature amplitude modulation (QAM) of the subcarriers is used.
Another disadvantage of the conventional OFDM transmission is its high peak-to-average power ratio (PAPR). As a result, OFDM signals cover a wide range of amplitudes but dwell mostly at small values. The disadvantages caused by this are twofold. As only the linear region of the amplifier can be used, high PAPR means low efficiency of the amplifier. On the other hand, OFDM signals have to be normalized to the conversion range of digital-to-analog (D/A) and analog-to-digital (A/D) converters for transmission and signal processing purposes. For a given quantization word length, a higher PAPR implies a lower signal-to-quantization-noise ratio.
The instant invention provides a new and simple multi-symbol encapsulated (MSE) OFDM system which employs a different type of cyclic prefix; instead of using one cyclic prefix for each OFDM symbol, a number of OFDM symbols are grouped together as a frame and protected by one single cyclic prefix. Two different frame implementations can be realized for different purposes, i.e., either to improve the bandwidth efficiency or to improve the robustness to synchronization errors and to reduce the PAPR of the MSE-OFDM system, as illustrated in FIGS. 2 and 8. These two different systems are named CP-reduced and FFT size-reduced MSE-OFDM system, respectively.
H. Sari, et al. in an article “Transmission Techniques for Digital Terrestrial TV Broadcasting,” IEEE Commun. Mag., vol. 33, no. 2, February 1995, pp. 100-109, and D. Falconer et al. in an article “Frequency domain equalization for single-carrier broadband wireless systems”, IEEE Communications Magazine, Volume: 40, Issue: 4, April 2002 Pages: 58-66, disclosed grouping multiple single carrier symbols into a frame followed by a cyclic prefix to facilitate frequency-domain equalization in single carrier systems. This approach essentially emulates the time-domain signal structure of the conventional OFDM system by providing a cyclic data frame at least several times longer than the channel response time; using a cyclic prefix in a single-carrier system for each symbol would be impossible because of a very short duration of the single-carrier symbol in a system having a comparable bit rate.
Encapsulating multiple OFDM symbols with a single cyclic prefix in one OFDM frame has not been disclosed heretofore; conventional OFDM systems already provide a cyclic frame structure enabling frequency-domain equalization. However, the multi-symbol encapsulation of OFDM symbols in a cyclic OFDM frame with a single cyclic guard portion would provide additional benefits compared to prior-art OFDM systems and the system of H. Sari et al., by potentially improving the bandwidth efficiency, enhancing system's robustness to synchronization errors and suppressing digitization noise through PAPR reduction.
It is therefore an object of this invention to provide a method for OFDM transmission wherein a high bandwidth efficiency is achieved by encapsulating multiple OFDM symbols in a frame with a single cyclical prefix.
It is another object of this invention to provide a method of a multi-symbol encapsulated (MSE) OFDM transmission with low peak-to average power ratio and enhanced tolerance to frequency synchronization errors.
It is another object of this invention to provide an MSE-OFDM system having high bandwidth efficiency.
It is another object of this invention to provide an MSE system for OFDM transmission having low peak-to average power ratio and enhanced tolerance to frequency synchronization errors.